The present invention relates in general to mass transfer columns and, more particularly, to liquid distributors used in such columns and methods of distributing liquid using the liquid distributors.
Uneven liquid distribution can lead to poor contact and mass transfer between ascending vapor streams and descending liquid streams in packing sections of mass transfer columns. Trough-like liquid distributors are commonly used in such columns to receive liquid from an overlying region and redistribute it in a more uniform manner to the underlying packing bed. These liquid distributors can be open at the top and bottom, with reduced spacing between the bottom side walls so that the liquid is funneled into a curtain or sheet-like flow as it exits the distributor. Liquid fed by a drip tube style liquid distributor will spread on the structured packing surface and will descend along the plate corrugations that extend in only one direction. The underlying layer of packing is rotated by 90 degrees and will spread liquid in the other direction, thereby achieving complete wetting of the packing surface. In another type of distributor, such as disclosed in U.S. Pat. No. 4,816,191 to Berven et al., the bottom of the distributor is closed and overflow holes are provided at preselected levels in the side walls of the distributor. The liquid exiting the overflow holes contacts splash baffles, which are positioned in spaced relationship from the distributor side walls. The liquid then flows down the splash baffles and drips from a lower drip edge into the packing bed along a drip-line, thereby requiring only one layer of structured packing to spread the liquid across the packing in both directions.
One problem associated with convention liquid distributors is the descending liquid may become entrained in the ascending vapor stream. In liquid distributors of the type disclosed in U.S. Pat. No. 4,816,191, entrainment may result from vapor flowing upwardly in the spacing between the splash baffles and the distributor side wall. In addition, the liquid is evenly divided between both splash baffles, thereby providing opportunity for entrainment as vapor ascends along the inner face of both splash baffles. In other liquid distributors, such as illustrated in EP 0282753B1, a second plate spaced from the splash baffle is used to shield the liquid as it splashes against and drains downwardly along the splash baffle. Because the lower edge of the splash baffle is spaced above the upper surface of the underlying mass transfer bed, the risk of liquid entrainment is still present as the liquid travels downwardly in the spacing between the lower edge of the splash baffle and the upper surface of the mass transfer bed. In addition, the spacing between the upper end of the splash baffle and the distributor side wall is closed by a flange that interconnects the splash baffle to the distributor side wall. This flange blocks upward escape of any vapor that has entered the spacing between the splash baffle and the second plate.
In one aspect, the present invention is directed to a liquid distributor for use in a mass transfer column having a mass transfer bed located in an open internal region within the column. The liquid distributor functions to uniformly distribute a descending liquid stream across the mass transfer bed for interaction with an ascending vapor stream. The liquid distributor has one or more optional feed boxes which deliver the liquid stream to at least one, and preferably a plurality of, underlying elongated troughs that extend across the column in parallel relationship. The troughs are spaced apart to permit vapor to flow upwardly in the spacing between adjacent troughs. Each trough has spaced apart first and second side walls that are interconnected by a floor. A plurality of liquid discharge holes are positioned in at least the first side wall, and preferably both side walls in an offset pattern. The liquid distributor further includes first and second splash baffles having lower portions forming a constricted discharge opening in a plane below the trough. At least the first splash baffle has an upper segment that is spaced outwardly from the first side wall and extends upwardly a sufficient distance in relation to the liquid discharge holes so that liquid exiting from the liquid discharge holes splashes against and runs down along an inner face of the splash baffle. The second splash baffle may also have a similar or identical upper segment spaced from the second side wall of the trough. At least the first splash baffle and preferably both splash baffles are vertically adjustable in relation to the trough so that they may be lowered onto the upper surface of the mass transfer bed. In this manner, the liquid is delivered from the constricted discharge outlet of the trough directly onto the upper surface of the mass transfer bed without having to travel through a vapor stream, which could cause entrainment of a portion of the liquid stream. In one embodiment, the liquid distributor contains a serrated drip edge that extends downwardly from the discharge outlet to deliver at least a portion of the liquid into the mass transfer bed, rather than just delivering it onto the upper surface thereof. Another advantage of the liquid distributor is the constricted discharge outlet blocks or impedes entry of vapor upwardly through the discharge outlet where it could interfere with the desired downward flow of liquid. The offset pattern of liquid discharge holes in the trough side walls allows for a more uniform distribution of liquid along the discharge outlet, thereby further reducing the opportunity for vapor entry into the discharge outlet. Any vapor entering the discharge outlet can flow upwardly through a significantly larger vapor outlet formed in the open spacing between the splash baffles and the trough at significantly lower vapor velocity, thereby minimizing the opportunity for liquid descending along the splash baffle to become entrained in the ascending vapor. Liquid that exits the trough through the overflow holes, or which simply spills over the top of the trough side walls, is able to enter this open spacing and is contained by the splash baffles for downwardly flow along the inner faces of the baffles. The mass transfer bed can comprise random, grid and/or structured packing elements, but preferable structured packing.
In another aspect, the invention is directed to a method of distributing liquid to the mass transfer bed using the liquid distributor described above to block or impede entry of vapor upwardly through the constricted discharge outlet formed between the splash baffles. Delivering the liquid directly onto the upper surface of the mass transfer bed while shielding the liquid from the vapor stream further reduces the opportunity for entrainment of liquid in the ascending vapor stream. The method includes permitting any vapor entering the discharge outlet to escape upwardly from the area between the splash baffles and the trough at significantly lower vapor velocity to minimize liquid entrainment from the splash baffle.
In a further aspect, the invention includes a method of installing the liquid distributor in the column by installing the troughs and then adjusting the splash baffles downwardly so that they are supported on the upper surface of the underlying mass transfer bed, thereby eliminating or reducing any gaps between the lower edges of the splash baffles and the mass transfer bed that can lead to liquid entrainment in the vapor stream. The adjustable nature of the splash baffles is particularly helpful in those instances where the troughs and the upper surface of the mass transfer bed are not parallel.